The present invention is generally related to a printhead for an inkjet printer and more particularly related to a printhead employing a particle tolerant ink feed filter of small dimensions to reduce particle blockages while maintaining a high rate of ink filling.
Inkjet printers operate by expelling a small volume of ink through a plurality of small orifices in a surface held in proximity to a medium upon which marks or printing is to be placed. These orifices are arranged in a fashion in the surface such that the expulsion of a drop of ink from a selected number of orifices relative to a particular position of the medium results in the production of a portion of a desired character or image. Controlled repositioning of the orifice-bearing surface or the medium followed by another expulsion of ink drops results in the creation of more segments of the desired character or image. Furthermore, inks of various colors may be coupled to individual arrangements of orifices so that selected firing of the orifices can produce a multicolored image by the inkjet printer.
Several mechanisms have been employed to create the force necessary to expel an ink drop from a printhead, among which are thermal, piezoelectric, and electrostatic mechanisms. While the following explanation is made with reference to the thermal ink expulsion mechanism, the present invention has application for the other ink expulsion mechanisms as well.
Expulsion of the ink drop in a conventional thermal inkjet printer is a result of rapid thermal heating of the ink to a temperature which exceeds the boiling point of the ink solvent to create a vapor phase bubble of ink. Rapid heating of the ink is generally achieved by passing a pulse of electric current through an ink ejector which is an individually addressable heater resistor, typically for 1 to 3 microseconds, and the heat generated thereby is coupled to a small volume of ink held in an enclosed area which is generally referred to as a firing chamber. One of the enclosing walls of the firing chamber is formed by the surface which is penetrated by the plurality of orifices. One of the orifices in this orifice plate is arranged in relation to the heater resistor in a manner which enables ink to be expelled from the orifice. As the ink vapor bubble nucleates at the heater resistor and expands, it displaces a volume of ink which forces an equivalent volume of ink out of the orifice for deposition on the medium. The bubble then collapses and the displaced volume of ink is replenished from a larger ink reservoir by way of an ink feed channel in one of the walls of the firing chamber.
It is desirable to have the ink refill the chamber as quickly as possible, thereby enabling very rapid firing of the orifices of the printhead. Rapid firing of the orifices results in the ability to achieve high-speed printing in an inkjet printer. Before the next firing of the heater resistor, the ink must have sufficient time to refill the chamber so that an undesirable variation in the size of the ink drop will not occur. Thus, one limitation on the speed at which printing may occur is related to the speed at which the firing chamber is refilled.
A problem that occasionally manifests itself in inkjet printheads is that of blockage occurring in an ink feed channel or in the orifice of the printhead. Microscopic particles can become lodged in the channel leading to the ink firing chamber thereby causing premature failure of the heater resistor, misdirection of ink drops, or diminished ink supply to the firing chamber resulting in greatly diminished ink drop size. A single orifice which does not fire an ink drop when it is commanded to do so leaves a missing portion from a printed character or creates a band of missing drops from a printed image. The end result is perceived as a poorer quality of printed matter, a highly undesirable characteristic for an inkjet printer. To resolve this undesirable result, others have suggested using spare or redundant orifices to eject ink in place of defective ink ejectors (see, for example, U.S. Pat. Nos. 4,963,882 and 5,640,183) or multiple inlets to the ink firing chamber.
Ink for inkjet printing is conventionally stored in a reservoir associated with the printhead mechanism. The apparatus for storing ink, such as a porous foam material or a sealed container, is known to shed particles, which can plug ink feed channels or ejection orifices. It has been observed that many of the particles are elongate, fibrous particles which are undesired products of the manufacturing process. The fibrous particles occasionally disengage from the ink containment apparatus and are carried by the ink to the printhead despite special cleaning processes and ink filtering which occurs prior to the ink entering the printhead (such as that described in U.S. Pat. Nos. 4,771,295 and 5,025,271). The filtering of elongate particles has been addressed in U.S. patent application Ser. No. 08/500,796, "Particle Tolerant Inkjet Printhead Architecture", filed on behalf of Timothy Weber et al. on Jul. 11, 1995, in which a plurality of outer barrier islands prevent elongate particles from reaching the ink feed channels or the ink firing chamber. Ink filtering has also been disclosed in U.S. Pat. No. 5,463,413 in which a plurality of pillars is arranged between the ink reservoir and the firing chamber, each pillar associated with the entrance to a firing chamber. The pillars are spaced apart by a distance less than or equal to the smallest dimension of the system, and are placed as close as possible to a common ink source to prevent particles from entering the firing chamber. The smallest dimension of the system is likely to be either the orifice bore diameter or the width of the passageway connecting the source of ink to the firing chamber.
As the dimensions of the orifices, firing chambers, and ink feed channels are reduced in order to provide improved printing characteristics, the size of the particles which, because of their small size, have passed through the ink feed channels and have been expelled from the orifices of previous designs, can now clog the printhead. In a design which employs orifices or ink feed channels having dimensions smaller than 20 .mu.m, particles and contaminants such as skin cells become candidates for lodging in the ink feed channel or orifice. Furthermore, particles such as skin and other biological cells are not rigid and therefore can deform and pass through a filter having a pore size equal to the smallest dimension in the printhead. Previous attempts to control and filter particles, while well suited for larger particles, do not solve the problem of clogging of the smaller passageways by the smaller particles.